The present invention relates to a device for the controlled delivery of an injectable liquid, in one embodiment, from a preferably substantially cylindrical liquid container. More particularly, the present invention relates to an injection device for administering a liquid containing a therapeutic or medical active agent, for example insulin for the treatment of diabetes, from a syringe body.
Injection devices of the aforementioned type are known for example from WO 95/09021. A syringe body for storing the liquid to be administered is sealed off at a rear end by a piston which can be slid axially in the syringe body, the liquid being delivered from an outlet at the front end of the syringe body when the piston is advanced towards the outlet. While in conventional injection devices comprising syringe bodies, the overall length must correspond to at least twice the maximum stroke of the piston which can be slid in the syringe body, the overall length can be reduced in injection devices of the aforementioned type by axially deviating a piston rod behind the syringe body, which drives the piston, away from its longitudinal axis. This is in particular advantageous in portable injection devices for autotherapy.
WO 95/09021 discloses a substantially incompressible, flexible piston rod which is tightly guided in a guide in order to prevent the piston rod from bending laterally away from its longitudinal axis. The piston is advanced by controlled rotation of a threaded rod, the rotational movement being converted into an advancing movement by means of a rotationally fixed but longitudinally slidable nut which engages with the outer thread of the threaded rod and is connected to the rear end of the flexible piston rod. This mechanism is hardly suitable for hand-operated injection devices without an energy supply of their own. In addition, the drive mechanism is comparatively elaborate.
An injection device operating according to the aforementioned basic principle is disclosed in WO 98/57688. When an operating button is depressed, a downward movement of a first gear rack is converted into an upward movement of a second gear rack which is coupled to a flexible piston rod. By rotating a dosage setting wheel, a stopper is adjusted within an adjustment range defined by maximum end positions, in order to set the dosage to be administered. The second gear rack can be maximally slid only by the distance pre-set by the stopper. Furthermore, a first spring is provided to reset the operating button, and a second spring which is provided in a dosage setting mechanism. The two springs are not coupled to the drive mechanism, in order to reset the latter and advance the piston for delivering liquid from the syringe body. Although the mechanism disclosed in WO 98/57688 can be operated by hand, it has a comparatively elaborate design.